1. Field
The present invention relates to an optical transceiver which monitors an input-light power which is input thereinto and outputs a monitored value.
2. Description of the Related Art
Recently, optical fiber has been used for a transmission path of the Internet traffic using Internet Protocol (IP), which has become widely used in a short time, and a transmission service which can transmit and receive a large amount of data is spreading.
An optical transceiver which transmits and receives a light signal via the optical fiber has been advancing in terms of MSA (Multi Source Agreement: industry standard) for downsizing and higher density of the optical transceiver, and for a stable supply of the optical transmission. Further, there are industry standardizations for downsizing and a shape such as SFF (Small Form Factor), SFP (Small Form Factor Pluggable) or the like.
SFP-MSA, a combination of the above standardizations, defines a DDM (Digital Diagnostic Monitor) function which monitors temperature, power supply voltage, bias current in LD (Laser Diode), output power (transmitting unit), and an input power (receiving unit), to thereby output the monitored information as digital values.
In general, the DDM function for the light-input power detects the current flowing through a light-receiving device PD (Photo Diode), and amplifies the same, performs an A/D conversion to convert the amplified current into the digital value, and then outputs the digital value. A result is obtained by calculating based on the output value using a predetermined calculation formula, whereby an user can check the intensity of the light which is input to the receiving unit.
A configuration of the light transceiver which outputs the monitored value using the above-described DDM function is described with reference to FIG. 6. FIG. 6 shows an example configuration of the light transceiver according to the conventional art. As shown in FIG. 6, the light transceiver receives a request for outputting the intensity of light via an apparatus (user circuit) connected therewith using two-wire interface. When light-input power is input to a light-receiving unit, a current following through PD is detected by an I/V converter. Then, the light transceiver, which has received the output request, converts the current into voltage. After that, the light transceiver converts voltage amplified by an amplifier (AMP) into a digital value via an A/D converter.
Further, a parameter is stored in a predetermined area of an EEPROM (Electrically Erasable and Programmable Read Only Memory), which is a nonvolatile memory which can delete/rewrite data by controlling electricity (voltage). After the description above, the light transceiver outputs the parameter and the converted value (monitored value) to the user circuit. The parameter stored in the EEPROM is set for each light transceiver during a production (preparation) of the light transceiver. For example, the parameter can be obtained by changing and performing A/D conversion on the light-input power several times, and calculating an approximate curve of the output monitored value.
The calculation of the intensity of light based on the above-described approximate curve is described specifically. As shown in FIG. 7, the monitored values which are A/D-converted and output are set as input conditions of the light input (W), and the parameters (A to E) in Equation. 1 are calculated and stored in the EEPROM in preparing the light transceiver. In operating the light transceiver, as shown in FIG. 8, the monitored values (output values (X) of the A/D conversion) output from the light-receiving apparatus and the parameters are assigned to variables in Equation (1) to thereby obtain the intensity of light (light-input power (Y)). FIG. 7 is an illustration of preparation of a light-input-power monitor according to the conventional art. FIG. 8 is an illustration of operation of the light-input power monitor according to the conventional art.Y=A×X4+B×X3+C×X2+D×X+E  (1)
Since demand for the dynamic range and accuracy differs depending on a user, the monitoring function needs to suppress errors. There have been disclosed various technologies for suppressing errors of the monitored value which is output by the optical transceiver (e.g., see Japanese Patent Application Laid-Open Nos. H11-183273 and H7-43211, and Japanese Patent No. 2560747).
In the conventional art described above, however, drift of the A/D converter is not considered. Specifically, as shown in FIG. 9, when the optical transceiver operates under temperature different from the temperature in preparing the optical transceiver, an A/D value of fluctuates due to each circuit error. FIG. 9 is an illustration of the errors of the monitored values according to the conventional art.
The A/D value output from the optical transceiver fluctuates depending on surrounding temperature even if the light-input power does not change. Since the parameters set in the EEPROM are constant values set in preparing the optical transceiver, the monitored values are to include errors when the output A/D value fluctuates. Thus, the monitored values include errors due to the surrounding environment, resulting in reduced accuracy of the monitoring.
Factors which cause the errors of the monitoring are driving condition of the light-receiving device (PD), temperature drift of the AMP and A/D converter, and the like. Particularly, when the light-input power is small, the monitoring is highly influenced by the temperature drift, quantizing errors, and the like of the A/D-converter, resulting in possibly reduced accuracy of the monitoring, and thus causing a problem in widening the dynamic range.